Ecological immunology in the Galapagos sea lion (Zalophus wollebaeki): Behavioural ecology meets conservation

Background/Question/Methods

Ecologists and conservation biologists are increasingly aware of the threat that infectious disease can pose to species and populations. In order to assess population-level risks and individual-level costs of pathogen exposure, we must understand the dynamics of immunity in the wild. This is the goal of the rapidly expanding field of ecological immunology, which brings together techniques from many disciplines to describe immunity as a life-history trait. Our study applies an ecological immunology approach to address the particular and peculiar conservation needs of the Galapagos sea lion (Zalophus wollebaeki). We use a combination of laboratory and field techniques to characterise the ontogeny of immunity in two colonies; one located on an uninhabited island, the other in a human settlement. Galapagos sea lions are uniquely tame, so in the urban setting they come into unusually close contact with dogs. This greatly increases the risk of pathogen transfer from dogs to sea lions, a fact that prompted IUCN to classify the Galapagos sea lion as endangered in 2008. Here we ask whether human influence has an impact on the ontogeny of immunity in the Galapagos sea lion and whether the activation of immunity incurs a growth cost.

Results/Conclusions

During the first 150 days of life, we find that animals from the human-impacted colony produce more immunoglobulin G than animals from the control colony (t_1,61=2.32, p=0.02); that males have lower serum concentrations of immunoglobulin G than females in both colonies (t_1,62=3.25, p=0.001); and that the increase in the inflammatory response to phytohemaglutinnin (PHA) is greater in the human-impacted colony (t_1,59 =2.25, p=0.02). Furthermore, the inflammatory response to phytohemaglutinnin (PHA) is negatively correlated with subsequent relative growth rate only in the human-impacted colony (t_1,59=2.51, p=0.01). Taken together, our results show that immune system activity is higher in the human-impacted colony and suggest that an increase in investment in immunity leads to a decrease in investment in growth. This has conservation implications for the Galapagos sea lion, as its two major threats are pathogen transfer from dogs and starvation due to climate fluctuations (El Niño). If, as our results suggest, there is an interaction between these threats mediated by a trade-off for energy within individuals, then pathogen pressure could affect sensitivity to starvation, and food availability could affect the chances of a disease outbreak.